Field of the Invention
The invention relates to optical fiber, and more specifically to methods and apparatuses for measuring one or more parameters of multiple cores at the same time in one or more optical fibers.
Description of Related Art
Optical interconnects and fiber optic links are becoming more and more prevalent not only for top-ranked super computer systems, but also for high performance data centers. This is due in large part to the superior bandwidth-distance product that they provide. This exponential growth in fiber volume has led to new and challenging design constraints for future systems. Consequently, increasing bandwidth per fiber, while minimizing further increases in link cost and power, is of paramount importance for the design of future optical interconnects. Among the promising solutions to increase bandwidth per fiber, while minimizing further increases in link cost and power, is to use Multimode Multicore Fibers (MM MMF), as demonstrated recently by Zhu & al and Lee & a. (See, e.g., “7×10-Gb/s Multicore Multimode Fiber Transmissions for Parallel Optical Data Links”, Benyuan Zhu, Thierry F. Taunay, et al., IEEE Photonic Tech. Lett., p. 1674-1649, vol. 22, (2010); “Multimode Transceiver for Interfacing to Multicore Graded-Index fiber Capable of Carrying 120-Gb/s over 100-m Lengths”, Lee, B. G.; Kuchta, D. M.; et al., N 23rd Annual Meeting of IEEE Photonic Society, (2010), p. 564-565, the teachings of which are incorporated by reference herein.)
One of the key issues which will determine the practicality and ultimately the performance of MM MCF is the ability to easily characterize fiber performance, and in particular the DMD of the individual cores of the MM MCFs. Differential Measurement Delay (DMD) method has been developed and standardized (TIA/EIA-455-220) as an accepted way to characterize the laser optimized high bandwidth MMF, which support high data rate digital transmission applications. In the DMD measurement, as shown in exemplary FIG. 1A, a short optical pulse with small spot size (e.g., ˜4 μm) is launched into the core 4 (e.g., ˜50 μm) of one MMF 6 end face 5 that is under test via a single core fiber 2. The resulting signal is measured at the output end face 7. Measurement is repeated as the spot scans gradually from the core center towards the cladding, and output pulses at the other MMF end face from each location are recorded and form the DMD pattern. The latter exhibits travel-time difference between the different modes ensemble excited for different probe positions, and the pulse broadening for the intermodal dispersion between mode groups. The spot size at MMF end face should be much smaller than MMF core size and can be originated from either a single mode fiber or image of the end of a single mode fiber.
While the current method to characterize MMF DMD is adequate for single core fibers, there are several major issues when it is applied to MM MCF. First, it is technically difficult to aligned launch fiber of image with the cores, which are not all located at the center of FUT. Second, measurement time for a MM MCF with N cores will be N times longer. Additionally, drift can occur owing to changes in temperature and other environmental variables over time and as caused by subjecting the FUT to a laser for an extended period of time.